As it is known, a progressive spectacle eyeglass compensates for insufficient accommodation of a wearer of this eyeglass, when said wearer is looking at objects that are situated at variable distances. For this purpose, the progressive eyeglass has an optical power that varies for different directions of gaze. More precisely, the progressive eyeglass has, at each point thereon corresponding to a direction in which the wearer is looking, an optical power that is adapted so that the wearer sees sharply an object situated in this direction at a given distance. Thus the wearer benefits from optimum vision through a progressive eyeglass if he acquires the reflex of observing the object through an appropriate part of the eyeglass, depending on the distance of this object. This reflex consists of spontaneously adapting the position of his head, essentially the vertical inclination of his head, so that the direction of his gaze passes substantially through the eyeglass at the point where the optical power is suited to the distance of the object.
When a wearer uses the same progressive eyeglass for a sufficiently long time, he has acquired a reflex for moving his head according to the distance of the object that he is observing, spontaneously integrating the particular variation in the optical power of this eyeglass. This variation in optical power parallel to the eyeglass, which is also referred to as the design of the eyeglass, is characterised by several parameters, including the progression length and the function of variation of the optical power along the meridian line of the eyeglass. This design may change between different progressive eyeglasses although these correspond to the same ophthalmic prescription. For this reason a wearer who is already equipped with an initial progressive eyeglass may need a period of habituation to adapt to a new progressive eyeglass, only because of the difference in design between the two eyeglasses.
In fact, every person has a reflex for spontaneously inclining his head according to the distance of an object that he is observing, even if he is not wearing spectacles. Such a reflex stems in particular from a physical posture that is more comfortable and causes less fatigue when it is maintained. The inclination of the head that is thus adopted by everyone when he is looking at an object in a fixed direction and at a fixed distance therefore varies between different persons. For this reason, a new wearer of progressive eyeglass, equipped with such an eyeglass for the first time, may also experience a problem when the design of this eyeglass does not correspond to his initial head inclination reflex.
Moreover, the inventors have observed that some people precisely incline their head according to the location of the object that they are looking at. In other words, they successively adopt positions for the head that are identical when they are looking at the same object successively under the same conditions.
Conversely, other people adopt head positions that vary for observation conditions that remain identical. In other words, these latter persons do not adopt again the same position of their head when they are looking at the same object under the same conditions at two different times.